The commercial production of phosphoric acid by the "wet process" generally comprises the digestion of ground phosphate rock containing apatite (3Ca.sub.3 (PO.sub.4).sub.2.CaF.sub.2) and/or tricalcium phosphate (Ca.sub.3 (PO.sub.4).sub.2). With dilute mineral acid (e.v. sulfuric acid) to produce a weak orthophosphoric acid solution in a calcium salt which is then separated from the solution by an appropriate technique. It has been found that if the sulfate ion concentration in the initial digestion tank is appreciable, the fluoride of the feed rock dissolves converting it to fluorosilisic acid which in turn dissolves the various clays and releases iron and aluminum and other impurities into the process stream.
Applicants have also noted the competing reactions which take place within the initial reactor vessel including solubilization of the phosphate by the phosphoric acid and the formation of gypsum by the sulfuric acid. This invention also deals with the removal of the sulfate ion in the phosphoric acid to a point where solubilization of the rock is both rapid and complete. In other words, the processing parameters have been adjusted to solubilize the greatest amount of rock with the minimum amount of calcium sulfate precipitation.